Desalination 190 (2006) 161–171

0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved

*Corresponding author.

Emulsion liquid membrane pertraction of L-lysine from diluteaqueous solutions by D2EHPA mobile carrier

Tahereh Kaghazchia*, Ali Kargaria, Reza Yegania, Abbas Zareb

aDepartment of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic),No. 424 Hafez Avenue, P.O. Box 15875-4413 Tehran, Iran

Tel. +98 (21) 66499066; Fax +98 (21) 66405847; email: kaghazch@aut.ac.irbDepartment of Biochemistry, Razi Research Institute, Hesarak, Karaj, Iran

Received 8 November 2004; accepted 23 June 2005

Abstract

The results of experimental studies on the batch extraction of L-lysine by emulsion liquid membrane are discussedand the capabilities of this method in the separation of solute from dilute aqueous solutions are shown. Studies onthe extraction equilibrium for organic and aqueous phases were performed. The experimental results showed thatthe degree of extraction was increased by an increase in the pH of the feed phase, the concentration of [H+] in theinternal phase, and concentration of the carrier in the organic phase. An optimum value of stirring speed wasachieved. The rate of water swelling as an undesirable phenomenon was also measured.

Keywords: Emulsion liquid membrane; Pertraction; D2EHPA; Carrier; L-lysine

1. Introduction

In recent years, liquid membranes (LM) havebeen widely used to study the ion transport againsta concentration gradient. The ion transportthrough an LM plays an important role in simu-lating biological membrane functions andseparation technologies because of high transportefficiency, excellent selectivity, and economicadvantages of LM. Fig. 1 shows a scheme of aconcentration gradient of transferred solute across

the membrane and the boundary layer in donorand acceptor solutions.

A number of successful research works, in-volving the transport of precious metal ions [3–6], rare earth elements [7], drugs [8–12], phenols[13–15], fructose [16–18], and the treatment ofseawater [19] and wastewater [20–23] through theliquid membrane, have been carried out. The trans-port kinetics of alkali metal ions [24], copper ion[25] and L-isoleucine [26] has also been studied.

Most of the biological products are diluteaqueous solutions that must be isolated from the

162 T. Kaghazchi et al. / Desalination 190 (2006) 161–171

Fig. 1. Concentration gradient of transferred solute acrossthe membrane and the boundary layer in donor and ac-ceptor solutions.

other chemical reagents such as substrates andnutrients in fermentation processes and by-products in chemical reactions. These separationprocesses usually are followed by a final purifi-cation such as chromatographic methods or crys-tallization, which sometimes, includes more than50% of the production cost. Different separationmethods such as ion exchange, chromatography,adsorption, many kinds of filtration, vaporization,reverse osmoses, electro dialysis, electrophoresisetc. could be used but in the case of low concen-tration biological feeds, the operating cost wouldbe very high.

Amino acids are important bioactive sub-stances. Study of the transport selectivity and ki-netics of amino acids through the liquid membraneare helpful for separation and concentration ofamino acids and for understanding the transportprocess of amino acids through the cell membrane[27].

Several papers have addressed the separationand transport of amino acids with liquid mem-branes. Ma et al. [28] studied the transport selec-tivity and kinetics of some amino acids such asL-phenylalanine, L-tryptophan, L-leucine, andL-isoleucine through di(2-ethylhexyl)phosphoricacid (D2EHPA)-kerosene bulk liquid membrane.

FDonor Solution

RAcceptor Solution

SLiquid

Membrane

XF

XFS

XSR

XSF

XR

XRS They claimed that the D2EHPA-BLM systemexhibits high transport selectivity for differentamino acids and the transport behavior is mainlyaffected by the lipophilicity constant of amino acidside-chain and the fraction of the protonated aminoacid in the feed solution. Oshima et al. [29] inves-tigated the liquid membrane transport of aminoacids such as L,D-tryptophan methyl ester hydro-chloride (Trp-OMe), L-phenylalanine methyl esterhydrochloride (Phe-OMe), L-tyrosine methyl esterhydrochloride (Tyr-OMe), and L-tryptophan (Trp)by a calix[6]arene carboxylic acid derivative andshowed that calix[6]arene hexacarboxylic acidderivatives are a useful carrier for amino acids ina liquid membrane transport system.

Wieczorek et al. [30] investigated the transportefficiency of five kinds of amino acids, namely,arginine (Arg), glutamic acid (Glu), phenylalanine(Phe), tyrosine (Tyr), and tryptophan (Trp),through the di-(2-ethylhexyl) phosphoric acid-tri-(2-ethylhexyl) phosphate supported liquid mem-brane (SLM), and a 56.2% of extraction efficiencyfor Trp was obtained.

Teramoto et al. [31], Itoh et al. [32], and Honget al. [33,34] explored the extraction and concen-tration of Phe and Trp by emulsion liquid mem-brane (ELM) containing D2EHPA as a carrier, res-pectively. Calzado et al. [35] studied the facilitatedtransport and separation of three kinds of aromaticamino acids through activated composite mem-branes (ACM) with D2EHPA as a carrier, andfound that the amino acid transport selectivitythrough these ACM followed a sequence of Trp >Phe > Tyr, regardless of a different amino acid–D2EHPA affinity sequence, Phe > Trp > Tyr.

Deblay et al. [36] developed a carrier-mediatedcounter-transport process for the successful sepa-ration of L-valine (Val) from fermentation brothsusing an SLM with Aliquat 336 as a carrier. Inthese studies, the effects of experimental parame-ters on the extraction efficiency of amino acidshave been investigated. However, the transportselectivity of multi-amino acids (more than fivekinds of amino acids) through the LM and the

T. Kaghazchi et al. / Desalination 190 (2006) 161–171 163

mechanism of stripping at the membrane-strippingsolution interface have seldom been reported.

Among the liquid membrane processes, emul-sion liquid membrane (ELM) has clear advantagessuch as a very larger mass transfer area and con-sequently a very much shorter time of extraction,applicability for extraction of very sensitive mate-rials such as enzymes, applicability of using ofexpensive and selective and even volatile extract-ants due to very small contents in the membranephase.

L-lysine is an essential amino acid for humandevelopment; probably the most limited in thefood chain and used for enrichment of cereals andfeeds. Few studies have been done on the separa-tion of L-lysine by liquid membranes [37].

In the present study, the extraction of L-lysinefrom dilute aqueous solutions by emulsion liquidmembrane was investigated. Nearly all ELM pro-cesses use liquid emulsifiers because of the dif-ficulties involving the solid emulsifiers. In thisstudy, a solid emulsifier (sorbitan monostearate)was used as an emulsifier for the first time; byproper selection of the membrane phase compo-sition, no shortcoming was observed.

2. Experimental

2.1. Apparatus

All experiments were performed at a constant

temperature of 25±0.2°C, using a water bathequipped with a P-type controller and an agitator.Extraction experiments were carried out in a cylin-drical glass container with a 94 mm diameter and120 mm height equipped with four round glassbaffles with a 5 mm diameter. The stirrer for theextraction experiments was made of glass with a21 mm diameter and 9 mm wide and had sixpitched blades with a 45° angle. The stirring driverwas a laboratory mixer model RE-166 from Ika-Werk Germany with variable speed 5.23–628 rad/s(50–6000 rpm). Fig. 2 shows a scheme of the ex-perimental set up.

The stirrer for the emulsion preparation wasmade from stainless steel and coated with LDPEto minimize contamination of emulsion. It had sixpitch blades with a 45° angle and 63 mm diameter.Samples were taken at predetermined time inter-vals by disposable sanitary syringes and centri-fuged by a laboratory centrifuge in order to sepa-rate the emulsion phase from the external phase.

In order to determine the concentration of L-lysine in the external phase, an UV visible spectro-photometer (Perkin-Elmer model 550) was used.pH measurements were carried out by a MetrohmpH meter.

2.2. Reagents

L-lysine (H2N-(CH2)4-CHNH2-COOH), iso-propanol ((CH3)2CHOH), n-decane (CH3-(CH2)8-

Fig. 2. A scheme of the experimental set up.

164 T. Kaghazchi et al. / Desalination 190 (2006) 161–171

CH3), hydrochloric acid (HCl) and ninhydrin,C9H4O3.H2O, were supplied by Merck Co. as ana-lytical-reagent grade. D2EHPA (di-2-ethyl hexyl-phosphoric acid), [C4H9-CH(CH2)-CH2]2HPO4,with purity of minimum 98% from BDH Co. andemulsifier Span®60 (sorbitan monostearate(C24H46O6)) from ICI, were used as received.Double distilled water was used throughout theexperiments.

2.3. Procedure

All experiments were performed at 25±0.2°C.Emulsions were prepared by dissolving the emul-sifier in a mixture of iso-propanol in n-decane(30% vol/vol) under light heating. Then, the inter-nal phase was added to the mixture and mixed for20 min at 6000 rpm to form a stable emulsion.Finally, the carrier was added to the emulsion andmixed for an additional 2 min. The ratio of mem-brane phase to internal phase was 3.

The external phases were prepared by dis-solving L-lysine in distilled water followed by pHadjustment. The external phases were placed intothe water bath for 20 min and then the emulsionwas added to the external phase with stirring. Theratio of feed to emulsion was 7. The external phasesolution was periodically sampled at certain timeintervals. The concentration of L-lysine in thesolution was determined after separation of theemulsion phase, by the UV spectrophotometer at570 nm.

3. Results and discussion

3.1. Extraction mechanism

L-lysine is an amino acid in which the sidebranches have positive charges at acidic mediumas H3N

+– (CH2)4 – CHNH3+ – COOH.

It is well known that this amino acid has dif-ferent ionic charges at different pH values. Threecommon dissociated states are as follows:

+ +3 2 4 3

+ + +3 2 4 3

1

H N (CH ) CHNH COOH

H N (CH ) CHNH COO H ,2.18pK

−

− − −

− − − +=

(1)

+ +3 2 4 3

+ +3 2 4 2

2

H N (CH ) CHNH COO

H N (CH ) CHNH COO H ,8.95pK

−

−

− − −

− − − +

=

(2)

+3 2 4 2

+2 2 4 2

3

H N (CH ) CHNH COO

H N (CH ) CHNH COO H ,10.53pK

−

−

− − −

− − − +=

(3)

For abbreviation, H2A2+, HA+, A and A– denote

different states of L-lysine according to the aboveequations.

In the extraction process, the carrier (HL) re-acts with the solute (A2+) at the organic and aque-ous phases interface and forms an oil soluble com-plex according to the following equation:

2+2 aq org 2 org

+aq

(H A ) (HL) (H AL )

(H )nn

n

+

+(4)

where, n is the coordination number of the reac-tion. Then the equilibrium constant for this reac-tion is expressed as:

+2 org aq

ex 2+2 aq org

(H AL ) (H )

(H A ) (HL)

n

nnK

⎡ ⎤⎡ ⎤⎣ ⎦ ⎣ ⎦=⎡ ⎤ ⎡ ⎤⎣ ⎦⎣ ⎦

(5)

Based on Eqs. (1)–(3):

+ +

1 2+2

3 31

HA H

H A

6.61 10 mol/dm

K

K −

⎡ ⎤ ⎡ ⎤⎣ ⎦ ⎣ ⎦=⎡ ⎤⎣ ⎦

= ×

(6)

T. Kaghazchi et al. / Desalination 190 (2006) 161–171 165

+9 3

2 2+

[A] [H ] 1.12 10 mol/dmHA

K K −= = ×⎡ ⎤⎣ ⎦

(7)

[ ]

+11 3

3 3

A H 2.95 10 mol/dm

AK K

−−

⎡ ⎤ ⎡ ⎤⎣ ⎦ ⎣ ⎦= = × (8)

In acidic medium with low pH values, [A–] isnegligible in comparison to other constituents inthe solution, then if [AT] is the total concentrationof different forms of L-lysine in the solution:

[ ] [ ]2+ +T 2A H A HA A A−⎡ ⎤ ⎡ ⎤ ⎡ ⎤= + + +⎣ ⎦ ⎣ ⎦ ⎣ ⎦ (9)

Then the total concentration can be simplified to:

[ ] [ ]2+ +T 2A H A HA A⎡ ⎤ ⎡ ⎤= + +⎣ ⎦ ⎣ ⎦ (10)

[ ]2+

1 22+T 2 +

2+1 2 2

2+

H AA H A

H

H A

H

K

K K

⎡ ⎤⎣ ⎦⎡ ⎤= +⎣ ⎦ ⎡ ⎤⎣ ⎦⎡ ⎤⎣ ⎦+

⎡ ⎤⎣ ⎦

(11)

[ ] 2+ 1 1 2T 2 2+ +

A H A 1H H

K K K⎛ ⎞⎜ ⎟⎡ ⎤= + +⎣ ⎦ ⎜ ⎟⎡ ⎤ ⎡ ⎤⎣ ⎦ ⎣ ⎦⎝ ⎠

(12)

1 1 22+ +

1H H

K K KM = + +⎡ ⎤ ⎡ ⎤⎣ ⎦ ⎣ ⎦

(13)

[ ] 2+T 2A H AM ⎡ ⎤= ⎣ ⎦ (14)

Applying Eq. (14) into Eq. (5) gives:

+2 org aq

ex

T aq org

(H AL ) (H )

(A ) (HL)

n

nn

MK

⎡ ⎤⎡ ⎤⎣ ⎦ ⎣ ⎦=⎡ ⎤ ⎡ ⎤⎣ ⎦ ⎣ ⎦

(15)

Eq. (15) can be rearranged to:

2 orgex

T aq

org

+aq

(H AL )log log

(A )

(HL) log

(H )

nMK

n

⎛ ⎞⎡ ⎤⎣ ⎦⎜ ⎟ =⎜ ⎟⎡ ⎤⎣ ⎦⎝ ⎠

⎛ ⎞⎡ ⎤⎣ ⎦⎜ ⎟+⎜ ⎟⎡ ⎤⎣ ⎦⎝ ⎠

(16)

or simply:

exlogY nX K= + (17)

The experimental runs were performed at25±0.2°C, using various carrier concentrationswithin the organic phase in the range of 0.3–0.9 mol/dm3. Samples were taken from the aque-ous phase until equilibrium conditions were estab-lished. The concentration of L-lysine in the aque-ous phase was measured. The concentrations ofthe latter within the organic phase were deter-mined from the differences between the initial con-centrations of L-lysine and those measured in theaqueous phase.

From Fig. 3, constructed by plotting

[ ]( )2 T aqlog (H AL) / (A )nM ⎡ ⎤⎣ ⎦ vs.

( )+org aqlog (HL) / (H )⎡ ⎤⎡ ⎤⎣ ⎦ ⎣ ⎦ ,

it may be observed that a linear curve has beenformed, having a slope of 0.24369 and interceptof –0.2755, then, n = 0.25 and Kex = 0.53027.

Consequently, the stoichiometric relation forthe reaction may be written as follows:

2+2 aq org 2 0.25 org

+aq

(H A ) 0.25 (HL) (H AL )

0.25 (H )

+

+(18)

0.25+2 0.25 org aq

ex 0.252+2 aq org

(H AL ) (H )

(H A ) (HL)K

⎡ ⎤⎡ ⎤⎣ ⎦ ⎣ ⎦=⎡ ⎤ ⎡ ⎤⎣ ⎦⎣ ⎦

(19)

166 T. Kaghazchi et al. / Desalination 190 (2006) 161–171

3.2. Effect of feed concentration

In order to evaluate its influence on the L-lysinetransport rate, the initial concentration of L-lysinein the feed solution was varied 1–5 g/l. The resultsare shown in Fig. 4. It is observed that at highinitial concentrations, the amount of amino acidthat crosses the membrane is relatively high, so itis easy to monitor the process by following theL-lysine concentration in the feed phase. On theother hand, lower initial concentration results ina lower amount of amino acid transported. It isreasonable to observe increases in extraction timeby increasing the L-lysine concentration in the ex-ternal phase. This is true for all extraction pro-cesses.

3.3. Effect of carrier concentration

It is well known that the rate of facilitatedtransport of the solute is directly affected by theamount of carrier that is present in the liquid mem-brane. From the results shown in Fig. 5, it couldbe observed that increasing the amount of carrierhas two effects: the viscosity of membrane phase,which limits the extraction rate, decreases by in-creasing the carrier concentration and hence thecarrier acts as thinner for the membrane phase. Atthe same time, increasing the carrier concentration

Fig. 3. Equilibrium extraction of L-lysine by D2EHPA.Fig. 4. Effect of external phase concenration on theextraction efficiency. Ci = 2M HCl; Cc = 0.4533M; Rew =1/7; pH = 2; 300 rpm.

Fig. 5. Effect of carrier concentration on the extractionefficiency. Ci = 2M HCl; Ce = 2 g/l; Rew = 1/7; pH = 2;300 rpm.

over a certain limit decreases the stability of theemulsion [38–42].

On the other hand, an increase in concentrationof the carrier in the membrane phase increasesthe ability of the membrane phase for extraction.Based on the emulsion phase, the best value ofthe carrier concentration was found to be about0.680M.

T. Kaghazchi et al. / Desalination 190 (2006) 161–171 167

3.4. Effect of internal phase concentration

Different types of acid solutions have beenreported as stripping solutions for liquid mem-brane processes using D2EHPA as a carrier. Sul-furic acid solutions [26,33,34] and hydrochloricacid solutions [30–32] were used for extractingamino acids using ELMs and SLMs, although itis reported that hydrochloric acid is a better strip-ping agent than sulfuric acid [28]. In this work,the effects of three concentrations of HCl strippingsolutions (1.0, 2.0 and 3.0 M) on the extractionefficiency are investigated.

The result that is shown in Fig. 6 indicates thatthe stripping efficiency increases with an increasein concentration of the stripping agents. Accordingto the two main factors affecting the strippingefficiency, i.e., the concentrations of H+ and thecoexistent anion (Cl–) in the stripping solution,this phenomenon may be explained as follows:high concentration of H+ is favorable to the strip-ping of L-lysine, however, high concentration ofcoexistent anion may be unfavorable for the strip-ping of L-lysine. According to the results shownin Fig. 6, 2.0M HCl is selected as the suitablestripping agent.

3.5. Effect of external phase pH

The extraction efficiency of L-lysine is ob-viously affected by the acidity of the feed solution.As shown in Fig. 7, the extraction efficiency in-creases with the increase of the pH value (at lowpH value), and then decreases with further increaseof the pH value (at high pH value). This may beexplained as follows:

The strong acidity is favorable to form H2A2+.

However, the strong acidity is unfavorable to theion-exchange reaction between H2A

2+ and thecarrier, according to Eq. (5), because the equilib-rium is shifted to the left side of the above extrac-tion reaction with an increase in H+ concentration.The final results are decided by the competitionof these two factors.

3.6. Effect of agitation speed

The effect of agitation speed on the extractionrate of L-lysine is shown in Fig. 8. The first obser-vation is that the percent extraction values at the1 min sampling time decrease as the mixing speeddecreases. The second one is that the maximumextraction value of each curve shifts to longersampling times as the mixing speed decreases.These observations can be explained as follows:

Fig. 6. Effect of internal phase reagent concentration onthe extraction efficiency. Cc = 0.4533 M; Ce =2 g/l; Rew =1/7; pH = 2; 300 rpm.

Fig. 7. Effect of external phase pH on the extraction effi-ciency. Ci = 2 M HCl; Cc = 0.4533 M; Ce = 2 g/l; Rew =1/7; 300 rpm.

168 T. Kaghazchi et al. / Desalination 190 (2006) 161–171

Fig. 8. Effect of the speed of agitation the extraction effi-ciency. Ci = 2 M HCl; Cc = 0.4533 M; Ce = 2 g/l; Rew =1/7; pH = 2.

as the mixing speed increases, two phenomenaoccur: (1) the globule size decreases and (2) therate at which globules are broken increases. Asmaller globule size will lead to a larger interfacialtransfer area between the feed and the membrane.This increased transfer area allows the extractionto occur at a higher rate, and explains the obser-vations noted above. On the other hand, the higherrate of shear and subsequent higher rate of break-age of globules at the higher agitation speedvalues, the higher rate of breakage allows moreleakage of the solute into the feed phase from theinternal phase [43]. Therefore, the optimum valuefor agitation speed was found to be 300 rpm.

3.7. Swelling of the membrane

Swelling is a measure of the amount of waterthat transfers from the feed phase to the emulsionphase. The values of swell reported in Fig. 9 werecalculated by measuring the volume of the emul-sion phase during the extraction run using thestirring-stop-stirring technique. The mechanismsof how swelling occurs are very complex.

Three possible mechanisms have been ob-served, as follows: the initial swell due to the ini-tial shear on the system, the swell due to an osmo-

Fig. 9. Time dependent of swelling property of the emul-sion. Ci = 2 M HCl; Cc = 0.4533 M; Ce = 2 g/l; Rew = 1/7;pH = 2; 300 rpm.

tic pressure difference between the internal phasereagent and the feed, and that due to the formationof new emulsion droplets during the extractionstep.

The swelling, however, can be controlled bytwo general methods. One is to formulate a mem-brane phase, which is not permeable to water. Theother method is to bleed off the internal phasefrom time to time (after demulsification) to keepthe water balance under control. The former is achemistry solution whereas the latter is a processremedy [38–45]. Existence of these high valuesof swelling is a common phenomenon in separa-tion by ELM, which has been reported by otherresearchers [39,40,43].

4. Conclusion

It was found that D2EHPA is a useful carrierfor L-lysine in emulsion liquid membrane transportsystem. The D2EHPA exhibited a strong inter-action with L-lysine and quantitative transport wasachieved. The forward extraction is based on theproton-exchange mechanism, and therefore thetransport rate could be controlled by changing thepH gradient between the feed and receiving

T. Kaghazchi et al. / Desalination 190 (2006) 161–171 169

phases. It is concluded that the D2EHPA is a goodcarrier for the transport of L-lysine.

Comparison of separation of L-lysine fromaqueous solutions by SX and ELM using D2EHPAin n-decane leads to the following results:• Under the ranges studied, the distribution ratio

increased with increasing pH at a fixed amountof D2EHPA concentration (Fig. 3).

• The suitable ELM parameters would be anaqueous external pH of 2.0, a Span®60 con-centration of 8 vol %, an aqueous internal HClconcentration of 2 M, a volume ratio of ex-ternal to W/O emulsion of 7/1 and that oforganic to internal phase of 3, a D2EHPA con-centration of 0.680 M and an agitation speedof 300 rpm.

• By proper selection of the extraction para-meters, the system is capable of almost com-pletely extracting the total L-lysine present inthe external phase (up to 5 g/l) within a fewminutes.

5. SymbolsA — Symbol for L-lysineCc — Concentration of carrier in the emulsion

phase, mol/dm3

Ce — Concentration of L-lysine in the externalphase, gr/dm3

Ci — Concentration of hydrochloric acid inthe internal phase, mol/dm3

K — Equilibrium dissociation constant,mol/dm3

Kex — Extraction equilibrium constantM — Parameter defined by Eq. (15)N — Speed of agitation, rpmn — Number of ligandsRew — The ratio of emulsion phase to the

external phase, vol/vol

Subscriptsaq — Aqueous phaseorg — Organic phase

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